Positron Emission Tomography (PET) imaging involves the creation of tomographic images of positron emitting radionuclides in a subject of interest. Conventionally, a subject (e.g., a human patient) receives a PET agent, e.g., a radiopharmaceutical, and then is positioned within a PET imaging system that includes a detector and detection electronics. As the PET agent decays, positively charged anti-electrons (positrons) are emitted. For commonly used PET agents the positrons travel a few millimeters through the tissues of the subject before colliding with an electron, resulting in mutual annihilation. The positron/electron annihilation results in a pair of oppositely-directed gamma rays with approximately 511 keV energy.
When the gamma rays impinge on the detector, the detector emits light, which is detected by detection electronics. The signals corresponding to the emitted light are processed as incidences of gamma rays. When two gamma rays strike oppositely positioned scintillators of the detector at approximately the same time, a coincidence is registered. The coincidences are processed to identify true coincidence events, which are binned and integrated to form frames of PET data that can be reconstructed as images depicting the distribution of the PET agent in the subject.
Another technique employed in medical imaging is Magnetic Resonance Imaging (MRI), which conventionally uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”) to polarize hydrogen nuclei in a subject's tissue so that the magnetic moments generally align along the direction of the main magnetic field. MRI systems conventionally include gradient coils that produce smaller amplitude (i.e. compared to the main magnetic field), spatially varying magnetic fields in response to an electric current control signal. Typically, gradient coils are designed to produce a magnetic field component that is generally aligned along the axis of the main magnetic field and that varies in amplitude with position along one or more axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength and concomitantly on the resonant frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are typically used to “spatially encode” the magnetic resonance (MR) signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. These coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal, which can be detected by the MRI system as MR data, and combined with multiple additional such signals may be used to reconstruct an MR image using a computer and known algorithms.
In recent years, hybrid or combined PET-MRI scanners have been developed so that PET and MRI images can be acquired using a single medical imaging scanner. While these conventional combined PET-MRI scanners can offer efficiencies over separate and individual PET scanners and MRI scanners, the combination of the PET and MRI scanners into a single scanner presents difficult challenges in realizing such efficiencies and ensuring high quality image acquisition.